Xylene-benzene conversion



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Sept. 6, 1949. R. J. LEE ET AL XYLENE-BENZENE CONVERSION Filed Jan. 22, 1947 KM c w s .w Q a mm mmm. mkxkmofifimk MVSQQ N mfi QEQMQ lnvenform Roberf J. Lee Hersche/ D Rae/fana' dffor'ney Patented Sept. 6, 1949 XYLENE-BENZENE CONVERSION Robert J. Lee, La Marque, Tex., and Herschel D.

Radford, Columbia, Mo., assgnors to Pan American Refining Corporation, Texas City, Tex., a corporation of Delaware Application January 22, 1947, Serial No. 723,648

' 3 Claims.

This invention relates to a process for the conversion of xylenes to toluene and is a continuation-in-part of our application for Letters Patent, Serial No. 480,912, filed on March 29, 1943, now U. S. Patent 2.416,184. In continuing our study of xylene conversion in the presence of a catalyst consisting essentially of anhydrous hydrogen fluoride, we have observed that when xylene-benzene mixtures are employed, the yield of toluene based on the xylene charge is related to the composition of the charging stock. More specifically, we have discovered that an unexpected and surprising increase in the yield of toluene based on xylenes charged is obtained over a relatively small range of Xylenezbenzene molar ratios, employing liquid hydrogen fluoride as a catalyst.

Accordingly, it is an object of our invention to provide a novel process for the conversion of mixtures of xylene and benzene to toluene in high yields. Another object is to subject mixtures of xylene and benzene containing amounts of xylene hereafter dened to the action of a hydrogen fluoride catalyst to produce a maximum yield of toluene based on the xylene charged.

A further object of our invention is to provide a process whereby maximum yields of toluene, based on xylenes charged, are obtained from mixtures of xylenes and benzene in the presence of a hydrogen fluoride catalyst by carefully controlling the ratio of xylene to benzene in said mixture.

Our invention comprises contacting mixtures containing between about 30 and about 50 mol per cent of a xylene and benzene with liquid hydrogen fluoride under conditions conducive to the transfer of methyl radicals from one xylene nucleus to nuclear carbon atoms in a benzene molecule or in another xylene molecule. The transfer of methyl radicals can usually be effected by agitating the charging stock with liquid hydrogen fluoride at temperatures between about 150 F. and about 450 F. under sufiicient pressure to maintain the reactants in the liquid phase. Pressures of about 100 to about 2000 pounds per square inch are ordinarily employed, e. g., pressures between about 1000 and about 1500 pounds per square inch. After the reaction has'proceeded to the desired extent, the reaction mixture is treated, e. g. by distillation or by coolmg and stratifying. to separate the catalyst.

which may be recycled to the conversion process with or without preliminary purification. The desired hydrocarbon products are then separated. e. g. by distillation, and unreacted feed stock and/or heavy ends may be recycled to the conversion process.

Although we prefer to use substantially anhydrous hydrogen fluoride as the catalyst, we have successfully employed aqueous solutions of hydrogen fluoride .containing as much as 25 weight per cent of water. In general, water tends to reduce the catalytic activity of hydrogen fluoride and to produce corrosive solutions. We have found commercial anhydrous hydrogen fluoride to be a satisfactory catalyst; no special precautions were found necessary topprotect liquefied commercial hydrogen fluoride from atmospheric moisture in our Operations. Commercial hydrogen fluoride may contain about 05% water, the average being 0.1 to 02%. It may sometimes be desirable to employ co-catalysts with the hydrogen fluoride, e. g. minor amounts of BFs.

In the conversion of xylene-benzene mixtures to toluene it is essential to use high catalyst-tofeed ratios. At least 100 weight per cent of hydrogen fluoride based on the total hydrocarbon charging stock should be employed. We have observed that the conversion of xylene-benzene mixtures to toluene increases markedly as the quantity of hydrogen fluoride catalyst is increased from about 100 to about 200 weight per cent, based on the total hydrocarbon charging stock. We prefer, therefore, to employ at least about 200 weight per cent of hydrogen fluoride catalyst based on the total weight of hydrocarbon charging stock in our process.

In converting a xylene-benzene mixture containing 45 mol per cent` of xylenes at about 350 Although temperatures between about '150 and about 450 F. can be employed in our conversion process, we prefer to use temperatures above about 200 F., preferably temperatures in the range of about 300 to about 450 F.

In our conversion process, sumcient pressure is maintained on the reactants so that they are present. almost wholly. as a' liquid phase in the reaction zone. The use of a liquid phase reaction system reduces pumping and storage costs, and facilitates the contacting of thev reactants with the catalyst.

In order to facilitate homogeneous and rapid reaction and to reduce catalyst requirements, it is advantageous to agitate the catalyst and react- 'ants vigorously. The agitation may be effected by any of the numerous well known means such as shaking, by the use of turbo mixers. orlfice mixers, by pumping the reactants rapidly in a closed cycle, injecting inert gases into the liquid reaction system and the like. Agitation may likewise be effected by circuating the reaction mixture through baiiied or packed towers or tubes containing chips or shavings of copper, chrome steels, nickel and the like.

Our invention may be operated as a continuous,

Semi-continuous or batch process. These meth- The irylene and benzene reactants can be A charged to the conversion equipment in a variety of ways. Thus, one or more of the hydrocarbon feeds may be dissolved in, or mixed with, liquid hydrogen fiuoride and charged to the conversion equipment, or the reactants and catalyst may be separately charged, or one reactant may be charged into a body of liquid hydrogen fiuoride and another hydrocarbon reactant.

Upon completion of the desired conversion, the hydrogen fiuoride may be separated from the hydrocarbon reactants and reaction products by distillation, stratification, or extraction processes. In general, we have found it expedient to separate the hydrogen fiuoride by stratiflcation and to reuse it in our conversion process. In another method of Operating, we may add ice, water, caustic, aqueous sodium carbonate, alcohols, ammonia and the like to the reaction mixture in order to remove the hydrogen fiuoride.

After separation of the hydrogen fiuoride, the hydrocarbons present in the reaction mixture may be fractionated to separate the desired product or products and unreacted hydrocarbons or by-products which may be recycled to the conversion process.

The conversion equipment may be made of or lined with copper, steel; stainless steel, Monel metal, nickel or platinum. When iron vessels are used, tarry residues are formed that are absent when copper vessels are used. for the same reaction.

Our invention will be better understood by reference to the following table and the appended figure. The experimental technique employed in obtainins the tabulated data was as follows:

The catalyst employed was commercial anhydrous hydrogen fiuoride. No extreme 4precautions were taken to exclude the possible absorpticn of moisture by the hydrogen fiuoride al- Weight per cent p-Xylene 21-23 m-xylene 40-44 o-xylene 15-18 Ethylbenzene 10-13 The aromatic content, as determined by the silver sulfate cataiyzed sulfonation procedure, was found to be 05.6 weight per cent. The experiments lwere conducted in either alloy steel or an illium bomb. The majority of these enperiments were performed in the illium bomb, in which the total volume of catalyst and hydrocarbon initially charged was approximately 65% of the actual capacity (739 ml.) of the bomb. The vcharge volume was adiusted so that thermal expansion of the reactants would Just fill the bomb at reaction temperature. This was done so that the vapor space would be negligible; hence the actual catalyst concentration in the liquidphase reactants would be equivalent to the amount of hydrogen fiuoride charged. Because of the desirability of adhering to this technique. the actual reaction pressure was moderateLv in excess of the normal vapor pressure of the system in some cases. This, of course, resulted from the development of hydrostatic pressure after the reactor contents had expanded to fill the allowable space. The system was equipped with a blow-out relief assembly to protect against exeessively high pressures. During reaction the bomb was secured in an electrically heated Jacket of a conventional design and the bomb was rocked through an angle of approximately 30 above and 30 below horizontal at the rate of about 58 4 low temperature (ca. 32 F.) could be maintained throughout the neutralization. Accordingly, the loss of hydrocarbon by volatilization was negligible, particularly since the greater part of the hydrogen fiuoride was neutralized before any appreciable quantity of hydrocarbon (upper phase) was displaced from the bomb. There was no determinable tar formation in the majority of these experiments. The reaction pressures were substantially constant-durlng fl-ny given run and essentially no gas was observed in the products. Therefore, apparently, the only loss of hydrocarbon was a small mechanical one, resulting from the relatively large volume of cracked ice and aqueous ammonia employed to scrub the hydrocarbon product and to absorb and neutralize the hydrogen fiuoride. Even this loss was sreatly reduced in many of the later experiments by rin'sing the bomb with isopentane and by extracting the aqueous layer from the neutralization of the reactants 'with lsopentane. Theiscpentane was subsequentiy removed from the hydrocarbon product by careful fractionation. By employlng this technique, product recoveries as high as 97% have been obtainable. Consequently it is Justifiable to express the product yields on a 100% recovery basis in these experiments. The tabulated data, therefore, are calculated on this basis for all of the experiments, except some individual cases (noted in the table) where a small amount In this series of runs, the mol per cent xylene in the hydrocarbon charge was varled from 100% xylenes to 20% xylenes with the remainder being benzene. All of these experiments were performed at a temperature of 356" F. with approxlmately 200% by weight of hydrogen iluoride based on total hydrocarbons charged. Run 10, in which xylenes alone were charged, shows the production of approximately 20 mol per cent toluene Wise-see footnote (2)).

of tar was formed. In these instances, the prod- 10 under these conditions. Simultaneously recovuct yield is based on the per cent hydrocarbon ered were more highly alkylated benzenes. As actually recovered. In these cases, also. methe mol per cent xylene in the benzene-xylene chanical losses are necessarily also disregarded charge was reduced. the toluene yi'eld calculated and the yields reported are. therefore, undoubtas per cent of the total hydrocarbon charge reedly lower than the true yield. mained substantially constant down to about 40% The composition of the hydrocarbon product 'xylene in the charge. Thereafter it fell off was determined by precise fractional distillaton sharply withlower concentrations of xylene in through a Podbielniak high-temperature superthe charge mixture. fractionating column containing a. 36" Hell-Grid The data obtained in the above group of expacking. The (197.6-248.0 F.) -fraction, subperiments have been recalculated to determine stantially all of which boiled at 230-231.8 F., was the yield of toluene based on xylenes present in taken as toluene. Refractive index measurethe charge and these results are shown in the ments for the toluene i'raction (nDflO; 1.4932- table and graphically in the appended flgure. 1.4950) and sulfonation data confirm the high It is very striking in examining the figure that aromaticity of this fraction. The identity of the the yield of toluene based on xylenes in the charge toluene product was verified by oxidation to benincreases rapidly to a pronounced maximum as zoic acid. The benzene, xylene and trimethylthe mol per cent xylenes inl the charge is inbenzene fractions showed similar high refractive creased or decreased to approximately the 40% indices and aromatic contents. Normally, durlevel. Thereafter this curve also shows a reducing these distillations, very little distillate was obtion in toluene yield. As the flgure indicates, tained between the boiling points of the respective maximum toluene yields based on xylenes aromatics. A high boiling hydrocarbon which charged are obtained when the charge contains had been carefully prefractionated to remove any between about 30 and about 50 mol per cent of constituents boiling below 392 F. was added to xylenes. Even more preferable is the use of the hydrocarbon product before distillation to charging stocksvcontaining between about 35 and permit complete recovery by distiilation of the abOilt 45 mOl D81' cent Of XyleneS in benzene. trimethylbenzenes. The trimethylbenzenes frac- The flgure carries a second curve showing the tion was considered to be that material fracyield of aromatic hydrocarbons in the trimethyltionated in the 320-347 F. range. The trlbenzene range (320-347 FJ. It is evident, that methyl-benzenes were characterized by refractive 40 throughout the composition range studied, deindex and boiling point. By careful fractionacreasing xylene concentration in the charge tion of a composite of the trimethyl-benzenes causes either no change in trimethyl-benzene fraction from several runs, it was found that yield based 011 XYlene charged AOr a reduction in pseudocumene (1.2.4 trimethyl-benzene), boiling trimethyl-benzene. The reduction in yield is 336.2-338 F., was the predominant isomer. definitely apparent as xylene concentrations fan The group of experiments whose results are debelOw 81101115 65 11101 Der cent; i. e.. as benzene conscribed in the table were carried out under subcentIatlOn is increased' This behavir iS exacfly stantially Constant Operating conditions to dethe ODDOSIPB Of that ShOWn by the toluene yield. termine the effect on toluene and trimethyl- A marked reductioninthe yield of trimethyl-benbenzene yields resulting from the addition of zenes, based on xylenes charged, is evidentin the varylng amounts of benzene with the xylene range of 30 to 50 mol per cent Xylenes in benzene. ehargmg Steek, If only disproportionation of xylene were oc- TABLE Conversion o :rylenes to toluene, benzene and polymethz/Zated-aromatics with. anhudroue hydroen fluorde [Moi per cent xylenes in charge as a variablo] Char Ick Average Approx. wt' Pep Reaclgeeno'imeraicukc MolHlillieent Ygfiuetol Algfx' Run Cent Xylenes Resction Pres- Cent tion MethylBen- Per dem on Yield No' Biglil Tirlp" slr'a HF d Ilurlse' Ben- Tolu- Trmeth llisltlslsagn xyleues wt' Mixture p' I zene one Xylenes benzeneys Loss ha' Pe' Cent 20 855 1,450 200 2 78.8 4.8 15.4 1.2 28 24 none 85 858 1, 500 200 2 58.8 18.0 18.8 4.1 8.8 48.25 18800 40 850 1,850 200 2 42.7 22.0 10.4 8.8 11.1 1 425 850 1,125 200 2 47.8 21.0 21.2 5.2 4.8 40.4 1 45 858 1,550 102 2 88.0 22.4 10.0 5.0 128 40.75 1 50 858 1,100 215 2 87.0 20.8 28.1 7.1 8.8 41.2 none 855 200 2 80.8 10.8 81.0 10.8 7.7 88 none 88.7 858 050 208 2 21.8 21.4 85.1 11.4 8.5 none 75 858 850 107 2 18.4 10.8 85.8 (o) 28.4 none 858 1,100 200 2 8.1 20.7 81.1 15.0 20.2 20.7 none Due to the formation of tal' in these reactions the yields of aromatic hydrocarbons are based on actual recovery. b Calculated to 100% liquid recovery assumng no loss of hydrooarbon in the experiment except distillation losses (unless designated otherd Based on total hydrocarbons charged.

anonse 7 currlng ln the reactlon lt would be expected that toluene and trlmethylbenzene ylelds would show roughly parallel behavlor. In xylene dlsproportionatlon. products other than toluene and trimethyl-benzenes are formed (e. g. benzene and more hlghly methylated benzenes). However, toluene and trlmethyl-benzenes represent prlmary products of xylene dlsproportionatlon and lt would be expected that as toluene yleld lncreases there should be an increase ln trlmethylbenzene yleld. If on the other hand, the benzene ln the reaetlon mlxture were to take part ln the reactlon and itself be methylated then one should get lncreased toluene ylelds accompanied by decreased ylelds of trimethyl-benzenes and the more hlghly alkylated benzenes. It can only be concluded. therefore, that when mlxtures of benzene and xylene are charged to the reactlon some methylatlon of benzene occurs.

Havlng thus descrlbed our invention. what we claim is:

We clalm:

1. The process which comprlses contactlng a mixture of a xylene and benzene contalning between 35 and 45 mol percent of xylene with at least about 200 weight percent of a catalyst con- 8 slstlng essentlally of hydrogen fluorlde at a temperature between about 300 F. and about 450' F. under sufllclent pressure to mafntaln the llquld phase for a reaction perlod sufhclent to produce a substantlal amount of toluene, and recovering tolucne from the reactlon mixture.

2. The process of clalm 1 whereln the amount of hydrogen fluorlde is about 200 pereent by weight of the mlxture of xylene and benzene employed as the charging stock.

3. The process of clalm 1 whereln the amount of hydrogen fiuorlde is about 200 percent by weight of the mixture of xylene and benzene employed as the charglng stock and the reactlon temperature ls about 360 F.

ROBERT J. LEE. HERSCHEL D. RADFORD.

REFERENCES CI'I'ED The following references are of record in the flle of this patent:

UNITED STATES PATEN'I'S Number Name Date 2.416,184 Lee et al Feb. 18, 1947 

